1. Field of the Invention
The present invention relates to an acceleration sensor using piezoelectric ceramic material.
2. Description of the Related Art
Various types of acceleration sensors using piezoelectric ceramic material have been proposed. Generally, in acceleration sensors, two types of sensitivities are used: voltage sensitivity and charge sensitivity. By increasing the charge sensitivity, the S/N ratio can be increased with respect to electromagnetic noise caused by external apparatuses and the circuit which affects the junction between the acceleration sensor and an amplifier, which is connected to the subsequent stage. On the other hand, by increasing the voltage sensitivity, the S/N ratio can be increased with respect to the voltage noise caused by the amplifier itself. Therefore, in order to increase the S/N ratio with respect to both external electromagnetic noise and noise inside the amplifier, both charge sensitivity and voltage sensitivity must be increased. That is, in a high S/N or highly sensitive sensor, a large amount of energy is generated, the energy being represented as xc2xd of the product of charge sensitivity and voltage sensitivity.
Japanese Unexamined Patent Application Publication No. 10-62445 discloses an acceleration sensor including three or more laminated piezoelectric ceramic layers, each layer having substantially the same thickness, and the layers being electrically connected in parallel. In this acceleration sensor, charge sensitivity can be increased by increasing the number of layers. In this acceleration sensor, however, by increasing the number of layers while the entire thickness of a sensing element is kept the same, the thickness of each layer is reduced and capacitance increases. Thus, the voltage sensitivity at each layer decreases accordingly. Also, the potential at an inner layer is lower than that at an outer layer. Therefore, by connecting these layers in parallel, the voltage sensitivity of the entire sensing element is the average of the voltage of each layer, and the entire voltage sensitivity is decreased as the number of laminated layers is increased. As a result, generated energy does not significantly increase.
Table 1 shows the sensitivity of two types of acceleration sensors: a two-layered type and a four-layered type. Herein, each acceleration sensor has a one-end-supported structure, the thickness of the entire sensing element is 0.42 mm, the free length except a supporting portion is 3.0 mm, and the width of the sensing element is 0.4 mm. As can be seen, the four-layered sensor has much higher charge sensitivity than that of the two-layered sensor, but voltage sensitivity is low, and thus the amount of generated energy in both sensors is the same. This table shows the characteristic in a state where each layer is electrically connected in parallel to each other, although the state is not shown in a figure.
In the above-shown characteristic, the entire thickness and free length of the sensing element are the same in the two-types of sensors, in order to compare the two sensors. In the characteristic of the shape including the thickness and free length, the following equations are satisfied:
charge sensitivity Q=kdxc2x7WL3/L; and 
voltage sensitivity V=kgxc2x7L2. 
If this shape is changed, the characteristic is also changed. Herein, L represents free length; W represents the width of the sensing element; T represents the entire thickness of the sensing element; g and d represent a piezoelectric constant; and k represents another coefficient. According to the above equations, the free length L should be increased in order to increase charge sensitivity and voltage sensitivity. Also, if the thickness T is decreased, the charge sensitivity increases and the voltage sensitivity is not changed, and thus, generated energy increases. However, the size increases by increasing L, and strength decreases by decreasing T. In this case, resonance frequency at a detecting portion decreases so that the acceleration at high frequency cannot be precisely measured. Thus, the size is limited.
Japanese Unexamined Patent Application Publication No. 9-26433 discloses a two-layered acceleration sensor having a one-end-supported structure. In this acceleration sensor, an intermediate electrode and a surface electrode are formed over the entire length thereof. When acceleration is applied to the sensor, stress is generated due to the acceleration and also a charge is generated so that an output signal is generated. In a one-end-supported structure, the stress is large at the vicinity of the supported portion of the sensing element and becomes smaller toward the free end thereof. In this state, the free end does not contribute so much to generation of a charge, and this state is equivalent to a state where only capacitance components are electrically connected in parallel. Therefore, voltage sensitivity of the entire sensing element is the average along the entire length. Also, the entire voltage sensitivity decreases compared to a case where an electrode is provided only near the supported portion, and thus, generated energy cannot be increased.
In order to overcome the problems described above, preferred embodiments of the present invention provide a highly sensitive acceleration sensor in which generated energy can be significantly increased without changing the free length and thickness thereof.
According to a first preferred embodiment of the present invention, an acceleration sensor includes a sensing element, and a pair of supporting members for supporting the sensing element at one end, both ends, or a central portion in the longitudinal direction thereof. The sensing element includes at least four laminated piezoelectric layers, each layer including a piezoelectric ceramic material. The four piezoelectric layers include a pair of first layers positioned at the center in the thickness direction and a pair of second layers sandwiching the pair of first layers. Electrodes are provided at the center in the thickness direction of the sensing element, between the pair of first layers and the pair of second layers, and on the outer surfaces of the pair of second layers. Cells formed by the first and second layers located on the same side with respect to the center in the thickness direction are electrically connected in parallel. The pair of first layers preferably have substantially the same thickness and the pair of second layers preferably have substantially the same thickness. The ratio of the thickness of each first layer to the total thickness of each first and second layer is about 62% to about 76%.
According to a second preferred embodiment of the present invention, an acceleration sensor includes a sensing element, and a pair of supporting members for supporting the sensing element at one end, both ends, or a central portion in the longitudinal direction thereof. The sensing element includes six laminated piezoelectric layers, each layer including a piezoelectric ceramic material. The six piezoelectric layers include a pair of first layers positioned at the center in the thickness direction, a pair of second layers sandwiching the pair of first layers, and a pair of third layers positioned on the outer side. Electrodes are provided at the center in the thickness direction of the sensing element, between the pair of first layers and the pair of second layers, between the pair of second layers and the pair of third layers, and on the outer surfaces of the pair of third layers. Cells formed by the first, second, and third layers located on the same side with respect to the center in the thickness direction are electrically connected in parallel. The pair of first layers preferably have substantially the same thickness, the pair of second layers preferably have substantially the same thickness, and the pair of third layers preferably have substantially the same thickness. The ratio of the thickness of each first layer to the total thickness of each first, second, and third layer is about 51% to about 62%. The ratio of the total thickness of each first and second layer to the total thickness of each first, second, and third layer is about 72% to about 87%.
According to a third preferred embodiment of the present invention, an acceleration sensor including a sensing element, and a pair of supporting members for supporting the sensing element at one end in the longitudinal direction thereof. The sensing element includes two or more laminated piezoelectric layers, each layer including a piezoelectric ceramic material. Electrodes are provided between the piezoelectric layers and on the outer surfaces of the piezoelectric layers. An electrode gap is provided in the free-end side of the sensing element, the electrode gap being a region where at least one of electrodes facing with the piezoelectric layers therebetween is not located. The ratio of the length of the electrode gap to the free length of the sensing element is about 20% to about 70%.
In the acceleration sensor according to preferred embodiments of the present invention, the sensing element preferably includes four laminated piezoelectric layers including a piezoelectric ceramic material, and the thickness of each first layer is larger than that of each second layer. More specifically, the ratio of the thickness of each first layer to the total thickness of each first and second layer is about 62% to about 76%. When the thickness of each of the four layers is the same as in the known art, the potential of the inner layer is lower than that of the outer layer even if the charge sensitivity is increased by increasing the number of layers. Thus, the voltage sensitivity decreases by connecting the layers in parallel, and the amount of generated energy does not increase. On the other hand, when the inner layer is thicker than the outer layer so that the potential in both layers is almost equal to each other as in preferred embodiments of the present invention, the generated energy is significantly increased. In an example, when the ratio of the thickness of each first layer was about 70%, the generated energy increased about 20% compared to the case where the first and second layer have the same thickness. Further, when the ratio of the thickness of each first layer was about 62% to about 76%, generated energy increased about 16% or more.
In the acceleration sensor according to preferred embodiments of the present invention, each of the first and second piezoelectric layers may be formed by a ceramic sheet having a predetermined thickness. Alternatively, each of the piezoelectric layers may be formed by laminating one or more ceramic sheets, each sheet preferably having the same thickness. In this case, if the number of ceramic sheets included in each first layer is preferably twice the number of ceramic sheets included in each second layer, the ratio of the thickness of each first layer is about 70%, and thus, maximum energy can be obtained. In this case, since the sensing element can be formed by using ceramic sheets having the same thickness, the thickness can be easily adjusted and manufacturing cost can be reduced.
In the second preferred embodiment, the number of layers included in the sensing element is preferably six and the thickness of the layers is large at the inner side and small at the outer side, so that the potential generated in the three layers is almost equal. Accordingly, generated energy can be increased. Also, by setting the ratio of the thickness of each first layer to the total thickness of each first, second, and third layer to about 51% to about 62%, and setting the ratio of the total thickness of each first and second layer to about 72% to about 87%, generated energy can be significantly increased compared to the case where all the layers have the same thickness.
In the sensing element having a one-end-supported construction according to the third preferred embodiment, by providing electrodes at the vicinity of a bonded portion of the sensing element, in other words, by removing the electrode at the free-end side of the sensing element, the voltage sensitivity can be significantly increased. That is, when acceleration is applied to the sensing element having a one-end-supported construction, large output is generated only from the vicinity of the bonded portion of the sensing element. Thus, the electrode at the free-end side of the sensing element, which has few effects on the generation of charge, is not necessary. Therefore, charge can be obtained from the portion generating a large amount of charge, and increased energy can be generated. By increasing the area of the electrode gap, the area of the electrode becomes small and thus the charge sensitivity decreases. However, the effect of the free end portion having a low potential becomes small, and thus, the voltage sensitivity increases. As a result, generated energy reaches maximum when the ratio of the electrode gap is in a predetermined range. According to an example, maximum energy is generated when the ratio of the electrode gap to the free length of the sensing element is about 50%. In this case, the amount of generated energy is larger by about 45% compared to the case when the electrode gap is not provided. Also, about a 20% or more increase in the generated energy can be realized when the ratio of the electrode gap is about 20% to about 70%. In this case, a connecting electrode for connecting the electrodes, which extend to the free end of the sensing element, is preferably provided on a side surface in the free-end side of the sensing element provided with the electrode gap.
Also, features of the first and third preferred embodiments may be combined. In this case, increased energy is generated due to the difference in the thickness of the first and second layers, as well as to the electrode gap. Therefore, a highly sensitive acceleration sensor can be obtained by the synergistic effect.
Further, the electrodes between the pair of first layers and the pair of second layers, the electrode at the center in the thickness direction, and the electrodes on the outer surfaces of the pair of second layers may be extended to the outside. That is, one end of each of the electrodes between the pair of first layers and the pair of second layers is positioned at the end surface of the sensing element supported by the supporting members, and the other end of each of the electrodes is positioned at a predetermined distance from the free end of the sensing element so as to form the electrode gap. On the other hand, the electrode at the center in the thickness direction and the electrodes on the outer surfaces of the pair of second layers extend from the vicinity of a proximal end of the sensing element supported by the supporting members to the free end of the sensing element. Also, the electrode at the center in the thickness direction and the electrodes on the outer surfaces of the pair of second layers are connected through the connecting electrode provided on a side surface in the free-end side of the sensing element.
In this case, the electrode at the center in the thickness direction and the electrodes on the outer surfaces of the pair of second layers can be connected by forming the connecting electrode on a side surface of the sensing element. With this configuration, a reliable connection can be ensured. Also, since the connecting electrode is provided at the electrode gap, the connecting electrode is not electrically connected to the electrode between the pair of first layers and the pair of second layers. Further, since the connecting electrode may be provided on a side surface of the sensing element, a complicated process is not necessary unlike in the case where the connecting electrode is formed on the end surface of the sensing element.
Other features, elements, characteristics and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments thereof with reference to the attached drawings.